The exhaust post-treatment device for treating a harmful component and a particulate matter in exhaust gas discharged from a cylinder is generally provided on the diesel internal-combustion engine (hereinafter, simply referred to as a “diesel engine”). The exhaust post-treatment device includes an exhaust purification catalyst for purifying the harmful component in the exhaust gas by catalyst reaction, a particulate filter mechanism (hereinafter, simply referred to as a “filter mechanism”) for collecting the particulate matter (hereinafter, referred to as PM) in the exhaust gas, and the like.
The exhaust purification catalyst includes, for example, a NOx occlusion-reduction catalyst for occluding nitrogen oxide (NOx) in the exhaust gas to reduce it to nitrogen. By supplying hydrocarbon as a reduction agent to the NOx occlusion-reduction catalyst, the occluded nitrogen oxide reacts with hydrocarbon and is reduced to nitrogen. On the other hand, the filter mechanism includes a diesel particulate filter (DPF) or the like for regenerating a filter by collecting the PM and burning the collected PM to discharge it as carbon dioxide. A method of burning the collected PM includes a method of heating the filter with an electric heater and a method of increasing the temperature of the exhaust gas flowing through the filter, and the method of increasing the temperature of the exhaust gas includes a method of increasing the temperature by adding hydrocarbon, which is the fuel, to the exhaust gas containing a large amount of oxygen.
In this manner, in the diesel engine provided with the exhaust purification catalyst as the exhaust post-treatment device, there is a case in which the fuel is added into the exhaust gas flowing toward the exhaust purification catalyst for supplying hydrocarbon as the reduction agent, such as a so-called rich spike. On the other hand, in the diesel engine provided with the filter mechanism as the exhaust post-treatment device, there is a case in which the fuel is added into the exhaust gas flowing toward the filter mechanism in order to regenerate the filter by increasing the temperature of the exhaust gas.
In order to add the fuel into the exhaust gas flowing toward the exhaust post-treatment device in this manner, the diesel engine in which an “exhaust fuel addition valve” for adding the fuel is provided on the upstream side of the exhaust post-treatment device in the exhaust passage aside from a fuel injection device for supplying the fuel into the cylinder is known (refer to the patent document 1, for example).
Recently, in the diesel engine, there is a case in which biological diesel fuel (hereinafter, referred to as biofuel) synthesized from vegetable oil such as canola oil and palm oil is used. The biofuel contains a larger amount of high-boiling point components than in a case of light oil, and is characterized in that this hardly vaporizes (less volatile) and the like.
There is a case in which such biofuel is mixed with the light oil at predetermined concentration and is used in the diesel engine. Therefore, in the control technique of the diesel engine in the patent document 1, the concentration of the biofuel contained in the mixed fuel is detected from an output of an air-oil ratio sensor provided on a downstream side of the exhaust post-treatment device (NOx occlusion-reduction catalyst/particulate filter), and a fuel additive amount at an early stage of the fuel addition is increased and the fuel additive amount at a latter stage of the fuel addition is decreased as compared to a case in which a predetermined light oil (reference fuel) is used, as the detected concentration of the biofuel becomes higher. Also, it is controlled such that difference between the fuel additive amount at the early stage of the fuel addition and that at the latter stage of the fuel addition is larger as compared to a case in which the predetermined light oil is used, as the exhaust temperature becomes lower.
Thereby, even when the mixed fuel having volatile (vaporizability) different from that of the predetermined light oil (reference fuel), behavior of the air-oil ratio in the vicinity of the exhaust post-treatment device (NOx occlusion-reduction catalyst/particulate filter) is made closer to the behavior in a case in which the predetermined light oil is added.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-177313